O manguezal como indicador natural de mudanças de paisagens entre terra-mar: um estudo no complexo estuarino do Rio Piranhas-Açu (RN/Brasil)

Abstract

Occurring between land-sea strips, the mangrove ecosystem plays important roles in the absorption and storage of CO2, in the maintenance of biodiversity, in water filtration and in the control of coastal erosion. Despite such relevance, this ecosystem has been identified as one of the most impacted, especially by climatic events and by the history of human occupation in the coastal zone. Considering this, a demand was created for research that expands the dialogue focused on the environmental quality of the mangrove from the identification, quantification, fragmentation, density, carbon capture and valuation of its biodiversity. Therefore, the objective of this research was to evaluate the spatial distribution, metric and density patterns of the mangrove that integrates the Estuarine Complex of the Piranhas-Açu River/RN, as well as the potential for atmospheric carbon capture and the monetary value of its mangrove vegetation. As methodological procedures, the following steps were followed: 1) Bibliographic survey; 2) Identification of mangrove features in 2008 and 2021; 3) Application of landscape metrics in the mangrove fragments of both years; 4) Based on the 2021 mangrove fragments, the following indices were calculated: Normalized Difference Vegetation Index (NDVI), Photochemical Reflectance Index (PRI), Improved Photochemical Reflectance Index (sPRI - positive PRI data) and the Carbon Flux Vegetation Index (CO2flux); and 5) Based on the 2008 and 2021 mangrove fragments, the monetary estimate was performed using the value transfer method. As a result, the mangrove mapping was carried out, where it was observed that in 2008 the vegetation occupied 3,168.1 ha (39.2%), the apicum 2,514.5 ha (31.1%) and the estuaries/tidal channels 2,396.3 ha (29.7%). In 2021, it was found that this vegetation occupied 2,838.3 ha (36.7%), the apicum 2,438.1 ha (31.6%) and the estuaries/tidal channels 2,444.9 ha (31.7%). As for landscape metrics, an increase in fragmentation was noticed, with the addition of 230 new patches in the last decade. Regarding the shape of the fragments, most were found to be irregular or not very compact, as well as a progressive indication of the edge effect with the increase of the perimeter-area ratio in the spots, over the time analyzed. Regarding the nearest neighbor metric, it was noted that the average distance between the fragments in 2008 and 2021 was 25 m and 19 m, respectively. Regarding the density and photosynthetic capacity of the vegetation to absorb CO2, it was observed that the NDVI indicated the occurrence of sparse, semi-dense and dense canopies; PRI showed full photosynthetic activity among plants, although with greater efficiency between semi-dense and dense canopies. This result was confirmed with the sPRI and the CO2flux, since the higher their values, the greater the ability to capture atmospheric CO2. In terms of the valuation of mangrove forests, it can be estimated that in 2008 the potential for providing services was quantified at US$ 793,634,144.79, while in 2021 it decreased to US$ 711,015,578.21, as a result of the decrease in area of mangrove fragments, which led to a total reduction of US$ 82,618,566.58.